Single-step conversion of synthesis gas (a gas comprising hydrogen and carbon monoxide and often referred to as "syngas") to dimethyl ether (DME) is very attractive as a route for indirect coal liquefaction, natural gas utilization, and production of synthetic liquid fuels or fuel additives. By converting synthesis gas to methanol and then further converting the methanol to dimethyl ether in the same reactor, the overall synthesis gas conversion is freed from the equilibrium constraint imposed by the thermodynamics of methanol synthesis alone. The prior art teaches a number of process schemes, many of which use gas phase, fixed bed reactors. The bifunctional catalyst system used in these schemes may be either a single catalyst with activity both for methanol synthesis and dehydration of methanol to dimethyl ether, or a mixed bed of methanol synthesis and methanol dehydration catalysts. However, liquid phase processes using a liquid phase reactor such as a slurry bubble column reactor (SBCR) offer significant advantages in heat transfer (which translates into process efficiency), operability and catalyst replacement. Therefore, it is highly desirable to use liquid phase technology for synthesis gas conversion to dimethyl ether. Liquid phase syngas-to-DME processes face one problem: there is a detrimental interaction between commercial CuO/ZnO/Al.sub.2 O.sub.3 methanol synthesis catalysts and most, if not all, effective methanol dehydration catalysts. This interaction is evidenced by loss of activity of one or both catalysts. ("A Novel Mechanism of catalyst deactivation in Liquid Phase Synthesis Gas-to-DME Reactions", X. D. Peng, B. A. Toseland, R. P. Underwood, Stud. Surf. Sci. Catal. Vol.111, 1997, pg 175). The prior art does not teach any methanol dehydration catalysts which demonstrate acceptable catalyst stability over the full range of commercially attractive conditions. Furthermore, the prior art does not teach single-particle, dual-functional catalysts for synthesis gas to dimethyl ether which are not subject to this detrimental interaction.
In the absence of a single catalyst or mixture of catalysts which can achieve acceptable catalyst life, the challenge to the industry is to devise process improvements to the liquid phase syngas-to-DME process which achieve acceptable catalyst life and stability using existing catalyst systems. Furthermore, it is necessary that such improvements not significantly lessen the overall process productivity and selectivity. The present invention is such an improvement and comprises introducing a methanol containing stream into the DME reactor such that the methanol concentration throughout the DME reactor is maintained at a concentration greater than 1.0%, generally between 4.0% and 8.0%. This keeps the reactor under a methanol rich atmosphere, which helps to increase the stability of the catalyst system.
The prior art teaches both gas and liquid phase processes for single step conversion of synthesis gas to dimethyl ether. Most of these do not concern themselves with catalyst deactivation. And more importantly, none of them teach that increasing the methanol level in the reactor is beneficial to catalyst stability.
U.S. Pat. No. 4,536,485 assigned to Haldor-Topsoe addressed the issue of deactivation of alcohol dehydration catalysts via coking or polymerization of hydrocarbons. This reference teaches a treatment whereby the most strongly acidic sites on the catalyst, which are responsible for the coking and polymerization, were selectively poisoned. However, this is a different deactivation mechanism from the interaction between the methanol synthesis catalyst and the methanol dehydration catalyst described above. Even dehydration catalysts which are not subject to coking or hydrocarbon polymerization still interact negatively with the methanol catalyst, causing deactivation of one or both catalysts.
Kokai Patent Application Number 3-181435 teaches a liquid phase syngas-to-DME process and cites as one of the benefits increased catalyst life due to greater resistance to poisons present in the feed and immunity to catalyst attrition issues associated with fixed bed processes. Loss of catalyst activity due to interaction between the methanol synthesis catalyst and the methanol dehydration phases is not acknowledged in this reference nor does this reference teach the introduction of methanol to the DME reactor.
Deactivation of syngas-to-DME catalyst systems has received some attention in the open literature. Dybkjaer and Hansen (Natural Gas Conversion IV, Studies in Surface Science and Catalysis, Vol. 107, p. 99, 1997, Elsevier Science B. V.) teach that a low ratio of carbon dioxide to carbon monoxide is important for the gas phase process they developed. In particular, this reference teaches that for the manufacture of methanol or DME, a desirable property of the synthesis gas is a relatively low ratio between carbon dioxide and carbon monoxide and that a high concentration of carbon dioxide leads to unfavorable equilibrium, high water concentration in the raw product, low reaction rate and increased rate of catalyst deactivation. This reference also does not teach that introduction of methanol to the DME reactor is desirable as a means of improving catalyst stability.
Xu et al. (Applied Catalysis A: General 149, 1997, p. 303-309) teach that increasing the concentration of hydrogen in the feed to a gas phase methanol-to-DME reaction improved the stability of their Pd/silica catalyst. Once again, the mechanism of deactivation which they addressed was carbon or hydrocarbon deposition. This in no way teaches any impact of the feed composition on the extent of deactivation resulting from interactions between methanol synthesis and methanol dehydration catalysts.
WO 96/23755 assigned to Haldor-Topsoe teaches a gas phase syngas-to-DME process where methanol is present in trace amounts (0.13% or less in the examples) in the overall reactor feed due to methanol carryover from the post-reactor separation process in which unreacted synthesis gas is separated from the reactor effluent and recycled back to the DME reactor. (The methanol concentration profile throughout the actual DME reactor is not known in this reference, however, it is known that the methanol concentration in the reactor effluent is 2.05% or less in the examples.) This inadvertent recycle of trace methanol as it relates to the methanol concentration in the DME reactor is not taught as a beneficial aspect of this gas phase DME process nor does this reference recognize the relationship between the methanol concentration in the DME reactor and stability of the bifunctional catalyst system. This is not surprising since the catalyst interaction problems are typically of more concern when the DME process is conducted in the liquid phase. Indeed WO 96/23755 teaches to convert byproduct methanol to DME in an additional reactor, instead of recycling it back to the syngas-to-DME reactor.